1. Field of the Invention
This invention relates to automatic focusing apparatus for photographic lenses and, more particularly, to distance measuring devices of the active type. Still more particularly, it relates to optical systems for use in distance measurement.
2. Description of the Related Art
Apparatuses for automatically focusing a photographic lens have been known in many forms, and various types of distance measuring devices for use in these apparatus have been proposed. One of these systems, for example, is called the passive type because focus detection is performed on the basis of information regarding the contrast of an object to be photographed, or the deviation of the actual position of an object image from the ideal position. Another system which projects a near-infrared light or ultrasonic wave onto an object to be photographed and receives the irregular reflection of the light or wave from the object to form a signal to be used in focus detection, is called an "active" type distance measuring device.
The latter or active type distance measuring device, because of the necessity of projecting the near-infrared light or ultrasonic wave, is comparatively disadvantageous from the standpoint of economy of energy, but has a superiority for detecting focus to an object of weak contrast with which the former or passive type distance measuring device can hardly deal.
The known apparatuses also can be classified in two other types, one of which, when measuring the object distance, uses light entering through all or part of the photographic lens and, therefore, called the TTL (Through-The-Lens) type. The other type which, besides the photographic optical system, has an optical system for focus adjustment is called the "external distance measurement" type. And, the external distance measurement type, because it is possible to secure a long enough length of the base line as compared with the TTL type, has an advantage that an accurate distance measurement becomes possible. The present invention relates to improvements of that type of automatic focusing apparatus which may be said by these classifications to be the active and external distance measurement type.
Next, the construction and operation of the general case of this active, external distance measurement type are described.
FIG. 1 shows a photographic lens of the so-called 4-component zoom type. In the drawing, reference numeral 1 denotes a first lens group for focus adjustment by variation of its axial position to vary the focusing (object) distance. Incidentally, as the first lens group moves to the object field, focusing can be effected down to shorter object distances. Here, letting the moved amount of the first lens group from the position at which it is sharply focused on an infinitely distant object (hereinafter referred to as "focusing movement") be denoted by X, and the focusing distance of the object by R, it is roughly safe to say that 1/R and X are in proportional relation. Therefore, from this relation, to focus on an object just in front of the lens by using the first lens group, the focusing movement has to be made extraordinarily large. Hence, it has been the common practice to limit the closest focusing distance to about 1.2 meters. In making a determination of this closest focusing distance, vignetting at the marginal zone of the image frame also is relevant. That is, the closest focusing distance is determined within the limitation that the corners of the effective image frame are not vignetted at any zooming position in the entire range of variation of the focal length.
Reference numeral 2 denotes a lens group for varying the image magnification. By varying the position of this lens group in the axial direction, the focal length is varied. Reference numeral 3 denotes a lens group for compensation. To keep constant the position of an image plane against the variation of the image magnification, this lens group cooperates with the magnification varying lens group when zooming. Reference numeral 4 denotes a diaphragm. Reference numeral 5 denotes a lens group for forming an image. Also, reference numeral 6 represents the image plane.
FIGS. 2 to 4 show the fundamental principle of a well-known one of the automatic focus adjusting methods employed in the active and external distance measurement type apparatus having the above-described 4-component type zoom lens.
In FIG. 2, a light beam for distance measurement issued from an infrared light emitting element 8 or the like is projected by a lens 9 onto an object 10 to be photographed, in this instance, a person. The projected light beam impinges on the object 10 and is irregularly reflected therefrom to a collector lens 11 which forms an image of the object on a photosensitive element 7. The image receiving surface of this element 7 is divided into two equal areas A and B, from which respective amounts of received light can be read. Now, assuming that the photographic lens is in focus, then an image of the object takes its place at the center of the entire area of the photosensitive element 7. Therefore, the outputs from the sensors A and B are almost equal to each other. In other words, as shown in FIG. 3(A), a spot of light is formed at the hatched position. If the object goes away to a position 10', the reflected light beam changes its course to a dot-and-dash line of FIG. 2, so that the light spot moves toward the area A, as shown in FIG. 3(B), where a relationship: "the output of sensor A &gt; the output of sensor B" is established. If the object moves to a position 10", the converse relationship: "the output of sensor B &gt; the output of sensor A" results.
It will be understood from this principle that by bringing the first lens group 1 for focusing shown in FIG. 1 and the position of the light sensitive element 7 of FIG. 2 into dependency with each other, an automatic focusing apparatus is formed in which the in-focus state occurs just when the outputs from both sensors A and B nearly or exactly coincide with each other.
FIG. 4 shows an example of the signal processing circuit for this type of automatic focusing apparatus. In the same drawing, to the photosensitive elements 7A and 7B are connected respectively amplifier circuits 12A and 12B, high-pass filters 13A and 13B for cutting off the D.C. component, and detector circuits 14A and 14B. Further, the outputs of the detector circuits 14A and 14B are connected to integration circuits 15A and 15B respectively. The light emitting element 8 is driven by a drive circuit 22. Responsive to a control pulse signal (not shown) output from a control microcomputer 19, the drive circuit 22 activates the light emitting element 8 to produce light in pulsating form.
The pulsated reflection from the object is received by the photosensitive elements 7A and 7B. The outputs of the elements 7A and 7B are amplified to a predetermined level in passing through the amplifier circuits 12A and 12B, then deprived of their D.C. components by the high-pass filters 13A and 13B, and further go to the detector circuits 14A and 14B. The synchronously detected signals each are integrated and smoothed by the integration circuits 15A and 15B. Their sum and difference are computed by an adder 16 and a subtractor 17. These values are inputted to a comparator 18 where they are compared with respective predetermined levels. When the sum of the integrated values has reached a predetermined value d.sub.A+B, focusing or a distance measuring operation is judged to be possible. The absolute value of the difference, too, of the integrated values is compared with the predetermined level to detect whether or not the image is in focus. When the image is out of focus, which of the near-focus and the far-focus is occurring is determined from the polarity of the difference signal. This determination result is inputted into the microcomputer 19. Depending on that result, it controls the drive circuit 20 for an electric motor 21 so that the lens is moved to the in-focus position.
Though the foregoing has shown the method of automatic focus adjustment in the active external distance measurement, other methods may be considered. For example, instead of the 2-area sensor, either a position sensor capable of detecting the absolute location of the image, or a CCD line sensor having many divided picture elements may be used to control the variation of the position of the front lens group in accordance with the result of detection of the object distance.
It is also to be noted that as the element operatively connected to the front lens group, besides the use of the photosensitive element, there are other methods of using the collection lens, or a transparent parallel flat plate arranged in the space between the collection lens and the photosensitive element.
FIG. 5 shows an operating system for the photographic lens including the above-described automatic focusing apparatus. In the figure, a tubular frame 23 containing the first lens group 1 for focusing has a geared portion 24 formed therein, and is operatively connected to the photosensitive element 7 or one of the others described above in a block 36 by means of a follower 25. A base barrel 26 carries the tubular frame 23 of the first lens group at its front end so that when the tubular frame 23 rotates about the optical axis, it is axially moved by a helicoid screw provided between it and the barrel 26. Thus, focusing is performed. A zoom ring 27 rotates in unison with a cam sleeve (not shown) fitted in the inner diameter of the barrel 26 to axially move the second lens group 2 for variation of the magnification and the third lens group 3 for compensation, while keeping predetermined relations therebetween. Thus, zooming is performed. The zoom ring 27 has a geared portion 33 formed in the outer surface thereof at the rear end. An electric motor 29 drives the diaphragm. A tube 30 holds a relay lens group 5. A zoom motor 31 rotates the zoom ring 27 through a gear 32. A focusing motor 34 rotates the tubular frame 23 of the first lens group through a gear 35. By this, the lens group for focusing moves axially. The aforesaid light emitting element 8 and the aforesaid projection lens 9 are mounted within the block 36. The aforesaid collection lens and photosensitive element are also mounted in the same block 36. A projected light ray 38 intersects with the extension of an optical axis A at a position 37.
FIG. 6 is a front view taken from the direction indicated by arrow A of FIG. 5.
FIG. 7 is a plan view showing the field of view of the photographic lens in the wide-angle end with a spot of light taking different positions for distance measurement when the object distance is 3 or 1.2 meters. FIG. 8 is similar to FIG. 7 except that the telephoto end is shown. The reason why the spot image for 3 meters lies always at the center of the area of the picture frame, (in other words, the target position does not change as the focal length varies) is that the position 37 shown in FIG. 5 is taken at 3 meters away.
Now, letting the distance between the spot images for the target position of 3 meters and the target position of 1.2 meters in the picture frame be denoted by X.sub.W in the case of FIG. 7 or by X.sub.T in the case of FIG. 8, and the focal lengths for the wide-angle end and the telephoto end by f.sub.W and f.sub.T respectively, the relationship: X.sub.T /X.sub.W =f.sub.T /f.sub.W is obtained. In an object plane at the distance of 1.2 meters, therefore, the discrepancy of the target point of the projected light beam from the point in axial alignment with the photographic lens remains constant. On assumption that, as shown in FIG. 6, the separation between the optical axes of the photographic lens and the projection lens is 50 m/m and the distance to the position 37 is 3 meters (3,000 m/m), then, from 50.times.((3,000-1,200)/3,000)=30, the discrepancy of the spot of light for 1.2 meters from that for 3 meters in the object plane of 1.2 meters in distance amounts to 30 m/m.
Such a phenomenon of deviating the target position from the line of sight due to the change of the object distance from the design value is called the "distance measurement parallax".
Several methods of removing such a distance measurement parallax have been proposed, one of which is exemplified in FIG. 9. The first lens group 1 in a position 44 focuses on an object 49. For this case, operative connections are made in such a way that the point of issuing light of the light emitting element 8 comes to a position 45 and the boundary between the two areas of the photosensitive element 7 to a position 48. As this condition changes by moving the object to a farther position 50, the position of the image of the spot on the photosensitive element 8 moves toward the photographic lens (upward as viewed in the figure), causing an unbalance between the outputs A and B. As a result, the near-focus state is detected and the first lens group is moved rearward to effect focusing to a farther object. At a position 43, the lens is sharply focused on the object at the farther position 50. During this time, as the focusing lens group moves from the position 44 to the position 43, the point of issuing light displaces from the position 45 to the position 46 and at the same time the light receiving boundary from the position 48 to the position 47.
The use of means for bringing both of the light projecting element and the photosensitive element simultaneously into such an operative connection with the lens group for focusing makes it possible to remove the parallax when the focusing distance is measured.
However, in the case of the apparatus shown in FIG. 9, unlike the case of the fixed passage of the projecting light beam shown in FIGS. 5 and 6,
(i) the linkage mechanism becomes complicated; PA1 (ii) the accuracy of distance measurement is lower than with the apparatus of the type having the fixed projecting light beam; PA1 (iii) for the purpose of compensating the above-described problems, each of the parts and assembling operations must be held to an increased degree of tolerance, or the size of the apparatus must be increased, PA1 (i) the power to be projected into the target zone diminishes because the projecting light beam passes through lens members constituting part of the photographic lens; PA1 (ii) for the purpose of putting the beam into the line of sight of the photographic lens, a half-reflection prism or other like expensive means becomes necessary, increasing the price; and PA1 (iii) the use of the half-reflection prism puts an obstacle in the way of advancing the technique of minimizing the size of the lens.
and other problems also arise.
To compensate the above-described drawbacks, a technique of making the projecting light beam coincident with the optical axis of the photographic lens by putting it in the TTL (Through-The-Lens) arrangement, too, has been proposed. But this gives rise to new problems as follows:
Meanwhile, for photographic lenses of today, the demand of shortening the closest photographable object distance is getting stronger. And, in such a lens type as has been described above in connection with FIG. 1, mutilation of the marginal rays of light gets appreciable particularly when the angular field is wide. With this in mind, it is known to provide a structure which enables the first lens group to further move forward with the limitation of its use to, for example, the telephoto end. However, if, as such a lens is combined with the automatic focusing apparatus described above, the path of the projecting light beam is left unchanged during zooming, and the inaccuracy of distance measurement due to the parallax shown in FIG. 8 is caused to increase largely. For example, when the usable focusing range is extended to 0.6 meters at the closest distance, the target area for distance measurement in the telephoto end gets completely outside of the field of view.
As is apparent from the foregoing, in the active and external distance measurement type of automatic focusing apparatus, the model of fixing the path of the projecting light beam has been suffering from parallax. This greatly affects the accuracy and reliability of focus adjustment and, in some cases, move the target zone for distance measurement out of the field of view of the photographic lens, depending on the focal length at the telephoto end and the object distance and further degrades the spatial relationship between the optical axes of the photographic lens and the projection lens.
Also, though proposals for improving this problem have been made, several of the above-described drawbacks remain unsolved.